Contribution of fiber orientation to enhancing dynamic properties of UHPC under impact loading

https://doi.org/10.1016/j.cemconcomp.2021.104108Get rights and content

Abstract

This paper investigates the contribution of fiber orientation to enhancing dynamic properties of ultra-high performance concrete (UHPC) under impact loading. Dynamic properties of 27 cylindrical core samples taken from prisms that have predominantly perpendicular, random, and parallel fiber orientations relative to the loading direction were tested. The Spilt Hopkinson Pressure Bar test method was used with strain rates of 160–290 s−1. Fiber orientation of samples after impact was evaluated using computed tomography scan. Test results indicated that the dynamic compressive strength, peak strain, and energy absorption capacity increased by up to 65%, 105%, and 295%, respectively, with the increase in strain rate. For a given strain rate, samples with predominant fiber orientations perpendicular and parallel to the loading direction showed the highest and lowest dynamic properties, respectively. Such spread in dynamic compressive strength, peak strain, and energy absorption capacity due to fiber orientation was up to 40%, 90%, and 135%, respectively. Fiber orientation coefficients along the direction of tensile stress were around 0.7 and less than 0.3 for samples with predominantly perpendicular and parallel fiber orientations, respectively. Good relationships were established to estimate the dynamic properties of UHPC given the quasi-static compressive strength, strain rate, and fiber orientation. A constitutive model to estimate the stress-strain curves of UHPC subjected to impact loading was developed. The model considers the influence of fiber orientation and strain rate. The model was successfully validated using the experimental data and was found to provide accurate prediction of the ascending portions of the stress-strain curves of UHPC under impact loading. The maximum spread between predicted and experimental dynamic compressive strength was limited to 10%.

Introduction

Ultra-high performance concrete (UHPC) is typically proportioned with high contents of cement, silica fume with high pozzolanic reactivity, well-graded fine sand, steel fibers, and high-dosage of superplasticizer to secure a very low water-to-binder ratio (w/b) [1,2]. Due to the low porosity of the cementitious matrix, UHPC can develop excellent mechanical properties and durability. Such superior performance has led to increasing use in the construction of large-span bridges, high-rising buildings, offshore structures, military facilities, and nuclear structures [[3], [4], [5]]. Apart from the quasi-static behavior, dynamic properties of UHPC are of great interest given the sensitivity of the material to loading rate, which can be especially elevated during explosions and impacts [6].

Strain rate, which refers to the rate of deformation under a given stress condition, can be employed to characterize the dynamic properties of concrete. It can be divided into three levels, including low (10−5–10−2 s−1), medium (10−1–102 s−1), and high (102–104 s−1) [7]. The dynamic properties of UHPC can be influenced by fiber type and volume [[8], [9], [10]], strain rate [[10], [11], [12], [13]], sample size [14,15], and quasi-static strength of the material [16,17]. Lai et al. [10] found that the dynamic properties of UHPC, including dynamic compressive strength, peak strain, and elastic modulus, can be significantly higher with steel fiber volume increasing from 0 to 4% and strain rate increasing from 24 to 96 s−1. Su et al. [17] used 3% nanomaterials (nano-CaCO3, nano-SiO2, nano-Al2O3, and nano-TiO2) to improve quasi-static strength of UHPC and found that the dynamic properties of UHPC can be improved with the addition of nanomaterials.

On the other hand, the quasi-static compressive strength and tensile strength of UHPC can be affected by fiber orientation [[18], [19], [20]]. Fibers can secure the highest reinforcing efficiency on the non-fibrous matrix for a parallel orientation relative to the direction of tensile stress. For the material under compression, tensile cracks are caused by Poisson's expansion. Therefore, fibers can be most effective in resisting crack propagation when their orientation is perpendicular to the direction of the compressive stress [19]. With the consideration of the effect of quasi-static strength on dynamic properties, fiber orientation can influence the dynamic behavior of UHPC. However, existing studies have mainly dealt with samples with random fiber orientation relative to the impact loading direction. The strain rate generated by explosions and impacts is generally higher than 100 s−1. Therefore, it is important to investigate the contribution of fiber orientation to the dynamic behavior of UHPC at high strain rates.

Various factors can improve the fiber orientation and lead to significant enhancement of mechanical properties. This includes the adjustment of the rheology of the UHPC mortar [18,21,22], mixing and placement methods [[23], [24], [25]], extrusion and formwork boundaries [26,27], and electromagnetic induction [28,29]. Besides, the fiber orientation shows flow-dependent characteristics, which is strongly affected by the flow of mixture. Huang et al. [20,30] reported that UHPC cast by the flow-induced method using an L-shape device can exhibit greater fiber orientation along the longitudinal direction of the cast prismatic samples compared to samples prepared by placing the UHPC at the middle of the molds that can result in random fiber orientation.

Generally, fiber orientation across UHPC samples is evaluated by image analysis of polished surfaces derived from areas near major cracks [20,[30], [31], [32]]. The evaluated fiber orientation does not fully correspond to that across the cracking location. Computed tomography (CT) scan can provide insights of fiber orientation in two- or three-dimensions without the need to polish cut surfaces and provide multiple surfaces to analyze [25,33].

Regarding the dynamic behavior of UHPC under impact loading, several models have been used to describe the stress-strain relationships of samples subjected to impact loading at different strain rates [8,10,11]. Rong et al. [11] employed the Holmquist-Johnson-Cook (HJC) model [34] coupled with the finite element method (LS-DYNA) to simulate the impact behavior of UHPC. This model is shown to provide good agreement between predicted dynamic behaviors and experimental results [11]. Lai et al. [10] applied the ZWT model [35] initially developed to evaluate the dynamic properties of polymers to investigate the dynamic behavior of UHPC at high strain rates. The model takes into consideration the visco-elastic characteristics of the material. The authors reported that the predicted stress-strain curves based on the ZWT model were very similar to experimental results, for UHPC samples subjected to impact loading at different strain rates [10]. Based on the continuum damage theory [36], Hou et al. [8] used a mesoscopic elemental damage-softening model to simulate the stress-strain relationship of UHPC. This model is shown to provide accurate prediction of the stress-strain relationships of UHPC under impact loading compared to the experimental results [8].

The three models discussed above [8,10,11] consider the influence of the strain rate on the dynamic properties of the material. However, the fiber orientation was assumed as random. The variation in fiber orientation relative to the impact loading direction can significantly affect the dynamic properties of the material. Such variation in fiber orientation was not considered in these models.

The objective of this paper is to investigate the contribution of fiber orientation to dynamic properties of UHPC subjected to impact loading. A total of 27 cylindrical samples were investigated. Cores were taken from prismatic samples that were cast using the flow-induced device in the perpendicular and parallel directions compared to the casting direction. Dynamic properties, including dynamic compressive strength, peak strain, and energy absorption capacity were tested using the Spilt Hopkinson Pressure Bar (SHPB) at strain rates ranging from 160 to 290 s−1. Fiber orientation of samples after the SHPB testing was evaluated using CT scan. A constitutive model that considers the contribution of fiber orientation and strain rate was developed to simulate the impact response of UHPC based on thermodynamics and damage mechanics.

This study includes two important novelties that have never been investigated before. The influence of fiber orientation on the dynamic properties of UHPC is studied and represents an important contribution of this paper. Different fiber orientations of samples relative to the direction of impact loading are manipulated using a novel flow-induced casting device. Furthermore, a constitutive model based on thermodynamics and damage mechanics is also developed to simulate the dynamic response of UHPC under impact. This model takes into consideration the contribution of fiber orientation and strain rate and can provide accurate prediction of the stress-strain response of UHPC under impact loading.

Section snippets

Materials, samples preparation, and test methods

The cement used had a compressive strength grade of 42.5 MPa, conforming to the Chinese standard GB 175–2007. Silica fume with a SiO2 content of 88% was used. The Blaine surface area of the cement and BET (Brunauer, Emmett, and Teller) surface area of the silica fume is 360 and 17300 m2/kg, respectively. Two types of quartz sand with particle size ranging from approximately 0.11–0.21 mm and 0.21–0.38 mm were employed. Steel fibers with 13 mm in length and 0.2 mm in diameter were added at 2%

Visual appearance

The mean quasi-static compressive strengths were 145.4, 127.6, and 108.3 MPa for samples having predominantly perpendicular, random, and parallel fiber orientation, respectively. Similar results were reported by Mansur et al. [19] where samples having predominant perpendicular fiber orientation showed higher compressive strength than that with predominantly parallel fiber orientation. Fig. 8 shows the failure modes of samples subjected to quasi-static loading for different predominant fiber

Establishment of constitutive model

As analyzed in Section 3.6, dynamic properties of UHPC can be significantly influenced by the fiber orientation of samples relative to the impact loading direction. Stress-strain relationships of samples underlie the analysis and simulation of the dynamic response of UHPC subjected to impact loading. This section aims to develop a constitutive model to estimate the stress-strain curves of UHPC subjected to impact loading that can take into consideration the influence of fiber orientation and

Conclusions

In this study, dynamic properties of UHPC having different predominant fiber orientations (i.e., perpendicular, random, and parallel, respectively) relative to the direction of impact loading were investigated. The SHPB test method was used to apply impact loading at strain rates ranging from 160 to 290 s−1. The dynamic properties of were correlated with the applied strain rate and fiber orientation of test samples. A constitutive model that takes into consideration the fiber orientation was

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 51578192) and the Program of China Scholarship Council (No. 201906120298).

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